1. Field of the Invention
This invention relates generally to physical vapor deposition (PVD) of thin films of materials from a target source onto a workpiece or substrate such as in the manufacture of integrated electronic circuits. More particularly, the invention relates to methods for improving the uniformity of target erosion and efficient utilization of target material in PVD processes, while reducing redeposition of particles on the target.
2. Background of the Related Art
Physical vapor deposition (PVD) or sputtering is a known technique used in the manufacture of integrated circuits. In PVD, a target of a desired coating material is bombarded by ions accelerated thereto to dislodge and eject target material from the target which is then deposited onto a substrate. The target and the substrate to be coated are generally placed in a vacuum chamber which is evacuated to and maintained at a pressure of less than about 10 milliTorr. Typically, a heavy inert gas such as argon is supplied to the vacuum chamber and a pumping system maintains the desired gas pressure in the chamber. A glow discharge plasma is created in the low pressure argon, at least partially ionizing the gas, by supplying a high negative DC, AC or RF potential to a cathode (typically the target) and grounding the chamber walls and an anode (typically the substrate). The glow discharge plasma is created in the space between the cathode and the anode, and is generally separated from the electrodes by a dark space or plasma sheath. Since the plasma itself is a good conductor, the plasma remains at essentially a constant positive potential with respect to the negatively biased cathode. This produces an electric field at the target that is substantially perpendicular to the exposed surface of the target. Thus, positive ions from the plasma are accelerated across the dark space onto the exposed surface of the target following trajectories substantially perpendicular to the exposed front surface of the target resulting in sputtering of the target.
In planar magnetron sputtering, the density of ion bombardment at the target surface is enhanced by producing a magnetic field in the plasma sheath adjacent the target that helps trap and deflect electrons near the target. Permanent magnets or electromagnets are located behind the target, or on the backing plate onto which the target is mounted, to produce a magnetic field parallel to the surface of the target. Plasma electrons spiral along these magnetic field lines and increase the electron density in these regions. The increased electron density contributes to additional gas ionization in these regions which leads to increased target bombardment and sputtering of the target between the poles of the magnets creating a pattern of target erosion.
Typically, a racetrack pattern is formed in the target between the poles of the magnets where the target erodes more rapidly. Physically rotating an off-axis magnet arrangement on the backing of the target may be used to control and alter the pattern of target erosion. The other areas of the target are eroded more slowly, and may not be eroded at all, thereby allowing redeposition or back sputtering of particles onto the target. Backsputtered particles tend to loosely adhere to the target surface on which they are deposited and tend to flake off over time. In addition, nonuniform erosion leaves a substantial portion of the target unused even when the areas between the poles have been completely eroded.
It is important that replacement of the target occur before the target is eroded down to the backing plate on which the target is attached. Particles sputtered from the backing plate material which reach the workpiece would seriously contaminate or destroy the workpiece. Consequently, a target is typically replaced before the target has been fully utilized leaving expensive target material unused. Since expensive target materials would thereby be wasted, it would be desirable if other means to assure uniform target erosion across the surface could be employed.
U.S. Pat. Nos. 5,320,728 and 5,242,566, which are assigned to the assignee of the present invention, teach the design of magnetron arrays to enhance target erosion uniformity. Other techniques such as the use of masks or blocking media placed over the target have been used to prevent redeposition of material onto the target. All magnetron targets, however, generally have areas of re-deposition on the target surface caused by target material becoming ionized either during or after sputtering and being deposited onto the target surface between the sputtering grooves formed by the high negative potential applied thereto. Masking surfaces do not prevent deposition of backsputtered particles on the masking surface and are not well suited to receive sputtered target material. Accordingly, when backsputtered material is deposited on the target or mask, the material tends to loosely adhere to the surface and can flake off and contaminate the chamber.
Therefore, there remains a need for a target source and sputtering system which enables efficient use of target material and which reduces the likelihood that back sputtered material will deposit on the target and become a particle source.